vite.config.ts import relative ts file cause error

[csr632]

Describe the bug

vite.config.ts import relative .ts file cause error.
vite.config.ts also doesn't support sourcemap.

Reproduction

Just change /playground/plugins/jsPlugin.js filename with .ts extension.
The playground will fail to start.

Add throw error in the file. You will find the sourcemap is broken when logging error.

Context

Rollup by default don't resolve extensions other than .js: rollup/rollup#1052 (comment)